Move the loading of surrounding diffs functionality into the model.

* `wp.revisions.log()` — temporary measure for logging based on `wp.revisions.debug`
* Return promises from functions that sometimes bail but normally return promises.

See #24425.

git-svn-id: http://core.svn.wordpress.org/trunk@24613 1a063a9b-81f0-0310-95a4-ce76da25c4cd